This invention relates to paint removal from metal members such as metal parts, objects and structures and more particularly, it relates to electrolytically assisted removal of paint from large structures such as bridge structures, tanks, ships, airplanes, automobiles and the like.
Prior methods of removing paint from large metal surfaces such as surfaces of steel bridge structures and holding tanks include abrasive blasting and chemical stripping. However, abrasive methods have the problem that they result in large amounts of the fragmented paint becoming airborne. This is particularly hazardous when the paint contains heavy metal compounds such as lead and chromate. Environmental regulations provide for stringent controls on the amount of metal such as lead that can escape into the atmosphere or onto surface soil and water. Contamination of water such as river water with paint is particularly troublesome because the metals in the paint can find their way into drinking water. To avoid this type of contamination when blasting, for example, attempts have been made to use enclosures around the structures to be blasted. However, such enclosures tend to be awkward and costly to use and often do not contain the abrasive and paint particles sufficiently well. Thus, hazardous quantities of the paint can still escape into the atmosphere and find their way to the soil and drinking water. Another area of concern is in the removal of paint from metal in confined areas, e.g., in the interior of a ship, where neither airborne particles nor fines are acceptable. In addition, abrasive blasting presents occupational hazards, and personnel must be protected from inhaling and contacting toxic paint constituents. Thus, in order to avoid contamination of the environment, abrasive blasting requires expensive precautions in an attempt to comply with environmental and health regulations. In the case of plastic media blasting of aircraft paints, chromate contaminates the blasting media, making disposal an environmental problem.
Another approach to removing paint coatings from metal structures involves the use of organic solvents or caustic solutions for chemical stripping. While the solvents can be effective in removing paint, they contaminate the environment upon evaporation and the escape of volatile organic compounds are restricted by law. Further, solvents have the problem of disposal after being used. The use of caustic solutions has the disadvantage that they are hazardous and require long and weather-dependent soak times to be effective. Thus, there is a great need for a system that avoids these problems.
In prior work, the use of electrochemical processes has been suggested for cleaning of metals. For example, Dunn U.S. Pat. No. 1,917,022 suggests the use of an electrochemical process for cleaning metal wherein the work is subjected to electrolytic action in a simple non-cyanide alkaline bath in the presence of metallic ions. According to Dunn, the work may be made either anode or cathode and in either case the dirt is subjected to three distinct cleaning actions; namely, the chemical detergent effect of the alkaline solution; the saponification and emulsification effect; and the mechanical action resulting from the liberation of gases at the work surface. Further, Dunn notes that while the metallic ion concentration may be inaugurated and maintained by the addition to the electrolyte of metal salts such as salts of lead, tin, zinc or cadmium, it is preferred to introduce ions by anodic action on the electrodes. According to Dunn, certain metals will have characteristic advantages and disadvantages. In the case of lead, lead peroxide forms at the anode and with the use of tin, metastannic acid forms. However, the Dunn reference has the disadvantage that it requires an alkaline bath and the addition of heavy metal ions such as lead or cadmium, further aggravating the environmental problem.
U.S. Pat. No. 3,900,376 discloses cleaning metal surfaces of elongated metal articles such as rods, bars, strips and wire. The metal articles are passed through an electrolyte such that a gas, e.g., hydrogen, is evolved at the metal surface. A high voltage is applied between the article and an inert anode such that the surface of the article in the electrolyte is completely covered by gas and vapor through which a discharge passes. However, the operation has to be carried out in the region of the current minimum of the current/voltage characteristic which occurs beyond the normal electrolysis regime as the voltage is increased. According to the patent, the high voltage and high current density cause substantial heat generation and the surface of the article is covered with a layer containing both hydrogen and steam. The discharge through the gas and vapor layer causes any scale on the article to flake off.
U.S. Pat. No. 2,765,267 discloses a process for stripping flexible films of resin which adhere to underlying metal bases to produce unsupported dielectric layers. The insulating layers are removed from the underlying bases by an electrolytic process in which the base metal is made the cathode in an electrolytic cell, and the insulating layer is forced off the base metal by the pressure of gaseous hydrogen at the junction between the metal and insulation, a distinctly different action than used in the present invention.
U.S. Pat. No. 3,457,151 discloses cleaning of an article made of conductive and nonconductive materials such as a printed circuit board, in an electrolytic bath and causing a current to flow in the bath between a cathodic element closely adjacent the board and an anodic element. The scrubbing action of the hydrogen bubbles generated at the cathodic element and at the conductive portions of the board cleans all of the surfaces.
U.S. Pat. No. 3,823,080 discloses an electrolytic process for removing a coating from a cathode ray tube mask member, and U.S. Pat. No. 4,439,289 discloses an electrolytic method for removal of magnetic coatings from computer memory disc using a sulfuric acid and glycerin solution.
ASTM Designation G95-87, "Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings" and ASTM Designation G8-90 "Standard Test Methods for Cathodic Disbonding of Pipeline Coatings" disclose test methods that cover accelerated procedures for simultaneously determining comparative characteristics of insulating coating systems applied to steel pipe exterior for the purpose of preventing or mitigating corrosion that may occur in underground service where the pipe will be in contact with inland soils and may or may not receive cathodic protection.
Other electrolytic cleaning methods are disclosed in U.S. Pat. Nos. 4,493,756; 5,104,501 and 5,232,563. However, it will be seen that there is still a great need for a process for removing paint coatings from metal members such as steel structures, automobiles and aircraft, which does not permit contamination of the environment with heavy metal components such as lead or chromium compounds contained in the protective coating.